Use entities in BackendClasses

pkg/compiler/lib/src/universe/function_set.dart

Index: pkg/compiler/lib/src/universe/function_set.dart
diff --git a/pkg/compiler/lib/src/universe/function_set.dart b/pkg/compiler/lib/src/universe/function_set.dart
index 3dc566ab5b6accbad4320a0014f38881895fa74d..90970db632a88752b0dd2cbb2049122b8c736d05 100644
--- a/pkg/compiler/lib/src/universe/function_set.dart
+++ b/pkg/compiler/lib/src/universe/function_set.dart
@@ -298,7 +298,7 @@ class FullFunctionSetQuery implements FunctionSetQuery {
           ClassEntity cls = element.enclosingClass;
           return [cls]..addAll(closedWorld.mixinUsesOf(cls));
         }).map((cls) {
-          if (closedWorld.backendClasses.nullImplementation == cls) {
+          if (closedWorld.backendClasses.nullClass == cls) {
             return const TypeMask.empty();
           } else if (closedWorld.isInstantiated(cls)) {
             return new TypeMask.nonNullSubclass(cls, closedWorld);